Midnight is no longer mid-night for individuals in the developed world. Many adolescents and young adults are awake and active at midnight. Artificial lighting, caffeine, and alarms have enabled this dissociation of sleep/wake+light/dark schedules from the external environment. In addition, many individuals have different schedules on work/school and non-work/non-school days, with later bedtimes, later wake times, and more frequent naps on non-work/non-school days. We and others have found correlations between irregular sleep/wake schedules and later circadian timing, poorer mood, poorer academic performance, and some health metrics, such as weight and psychiatric disorders. These, however, are correlations and not causal links. Identifying mechanisms by which modifiable behaviors, such as variable sleep timing, impact physiology is vital to developing effective educational or treatment-based strategies. We propose an experiment to test the effects of irregular and regular sleep/wake schedules on circadian timing, sleep, cognitive performance, mood, and learning. We will study a young adult population, as this is a time in human development when variable sleep schedules are prevalent and the consequences of irregular schedules on circadian timing, sleep propensity, performance, mood, and learning may be pronounced. The results from the proposed work will allow us to determine if it is the irregularity of the sleep/wake light/dark (SW/LD) schedule that delays circadian timing and disrupts performance, mood, and learning. Using our Sleep Regularity Index (SRI, a quantitative measure of sleep regularity), we will identify and select individuals with Irregular and Regular sleep/wake schedules based on the individual's sleep-wake pattern during three weeks of outpatient screening at home. Individuals from each group will then be randomly assigned to either an Irregular or Regular SW/LD schedule for an inpatient assessment. Assessments of circadian phase, sleep propensity, cognitive performance including vigilant attention, sleep-dependent and long-term learning, and mood will be conducted during the inpatient portion of the study. Our findings will help define physiological links among multiple physiological systems, including sleep/wake, light-sensitive processes (e.g., circadian rhythms, alertness, hormones), cognitive performance, and mood. Reducing scheduled sleep variability has no pharmacologic side-effects and may have additional health benefits. Designing effective therapeutic strategies for reducing variability requires an understanding of the physiological and environmental factors that may mediate and reinforce irregularity. Our results will help to elucidate these mechanisms and will be applicable to high school and college students, as well as adults working shift or night work or experiencing jet-lag. The findings may be utilized in public health forums, including educational campaigns about sleep timing and its effects on circadian rhythms, performance, learning, mood, and safety.